OFDM transmission is a promising access scheme in the field of wireless communication because of the advantageous features in the multipath propagation environment. In OFDM transmission, a data channel (or a sequence of symbols) to be transmitted is associated with multiple subcarriers selected so as to be orthogonal to each other, and is subjected to inverse Fourier transform and application of a guard interval, prior to being transmitted as OFDM symbols. At the receiving end, the guard interval is removed from the received signal, and Fourier transform is performed to extract information from each of the subcarriers. Then, the transmitted data channel is recovered.
The wireless receiver receives a control channel, as well as the data channel. The control channel includes a pilot channel containing priori known symbols, a common control channel used to transmit common information to all wireless receivers in the system, and an individual control channel used to transmit an individual data item to a wireless receiver. Dedicated resources are allocated to the control channel, which control channel is multiplexed with the OFDM symbols transmitted from the wireless transmitter. The wireless receiver extracts the control channel, including the pilot channel, from the received OFDM symbols, and performs channel estimation and securing of synchronization timing. This type of wireless transmission using an OFDM scheme is described in JP 2001-144724A.
FIG. 1A and FIG. 1B are schematic diagrams illustrating a control channel multiplexed with a data channel. In FIG. 1A, the control channel is frequency-multiplexed with the data channel by allocating a certain band of the spectrum. In FIG. 1B, the control channel is time-multiplexed with the data channel by allocating a certain time slot to the pilot channel.
It will be more and more required for this field of technology to catch up with the increasing moving speed of mobile terminals, expansion of the available frequency band, and shift to higher ranges of frequency, from the viewpoint of providing high-quality services. Accordingly, it is required to provide communication services capable of sufficiently meeting a rapid change in the signal level along the time and frequency axes occurring in the communication environment.
As illustrated in FIG. 1A and FIG. 1B, a control channel (such as a pilot channel) is inserted only in a specific domain along the frequency axis or the time axis. For this reason, if the signal level abruptly changes in a domain in which the pilot channel is not inserted, satisfactory channel estimation cannot be performed. In this case, the function and the objective of the pilot channel cannot be achieved sufficiently. This means that the resources allocated to that control channel are not being efficiently used.
In addition, since dedicated resources are allocated to the control channel, the resources to be allocated to the data channel decreases. If the resources dedicatedly allocated to the control channel cannot be used efficiently, allocating the dedicated resources to the control channel, while decreasing the resources to be allocated to the other channels, becomes meaningless.